Heat pumps using existing refrigerant-cycle technology absorb heat from an outside environment with a fixed-size evaporator in which a refrigerant converts from liquid to gas. They compress the gas, which reverts to liquid form in a condenser and gives up heat for space heating or for domestic hot water. Performance of these conventional heat pumps suffers from their inability to accommodate wide variations in the amount of heat available to the evaporator from the environment due to changes from summer to winter and from day to night.
Evaporator size for a conventional heat pump is dictated by such factors as the desired heat pumping capacity of the system, refrigerant characteristics and the highest outside temperature expected. As outside temperature falls it is a natural result of fixed-size evaporator performance that the heat output of a conventional heat pump falls accordingly--just when most needed. (This effect is discussed and charted in more detail later in this application.)
Skilled workers in the heat pump field have failed to discover a useful improvement which would enable a heat pump to continue steady performance during periods of falling temperatures instead of showing the customary fall-off in output.
Another deficiency of conventional heat pumps is that they slowly become non-functional when moisture in the air moving over cold evaporator coils deposits onto them as frost, which accumulates and impedes air movement and reduces heat absorbing capability. Special means must therefore be included to overcome the frosting problem. Such means commonly take the form of furnishing heat to the evaporator to melt frost away. Meeting this requirement entails a triple loss: (1) Heat pumping capability is reduced as frost forms, (2) Heat pumping is partially or entirely suspended while frost is melted and (3) Heat used to melt frost is lost to the environment--after the heat pump has worked to bring in heat from that environment.
Existing heat pump technology can't satisfy every goal for system optimization, so what results is a conventional heat pump system, plus a backup heat source for the coldest periods, plus added-on provisions for melting evaporator frost, plus safety devices to shut the system down if warm weather conditions overstress the heat pump. The fundamental problem is that a heat pump designer can't satisfy every requirement for evaporator sizing. A large evaporator is needed on cold days so that a large amount of heat transfer can keep the pressure of evaporated refrigerant up to the value required for normal system operation. But a large evaporator's output on warm days will be at temperatures and pressures far above those the evaporator and compressor can tolerate. When a designer specifies a smaller evaporator that won't harm the rest of the system on hot days, it frost up on cool days. The heat pump designer is compelled to provide a crutch for the system--a way to defrost the evaporator, with the added complexity and loss of efficiency such a requirement brings to the system.
Strong needs have existed for many years for heat pumps that would not have these two major problems and other deficiencies. Heat pumps became commercially available in the 1950's, but they are still not widely used because of their obvious drawbacks. Even the most skilled workers in heat pump field have treated the symptoms instead of the causes. Instead of solving the basic problems by devising a new and improved heat pump system able to operate over an extreme range of outside temperatures and without being hampered by frost deposition, they have devised clever methods for melting ice and frost from evaporators, they have devised protective measures caused by fixed-size evaporators absorbing too much heat, they have used evaporators whose maximum size was dictated by maximum pressures to be experienced, and they have made sporadic attempts to do something about evaporator size inflexibility. But even the emphasis added by the energy crunch instituted in 1973 by the oil embargo has not resulted in solutions to these basic difficulties.
My invention, admittedly appearing simple and obvious, aims at a solution to these basic difficulties brought about by a combination of improvements that apparently had not been obvious to workers in the heat pump field who were concentrating on alleviating the symptoms of being hampered by frost and being unable to pump heat from low temperature environments.
Taplay(U.S. Pat. No. 4,352,272, Oct. 5, 1982), for example, stating that " . . . the coils of an ambient air absorber must be defrosted periodically . . . ," disclosed a mode of operation in which defrost heat is applied to a primary ambient air heat absorber both from an electrical heater and also from hot refrigerant gas while heat pump operation goes on at reduced capacity using a secondary heat absorber " . . . which can be of same type, or of the type obtaining heat from solar insolation, water, ground heat, etc. . . . "
Lindahl et al (U.S. Pat. No. 4,122,686, Oct. 31, 1978) disclosed a way to work the defrosting chore by isolating one segment of a multi-segment evaporator and sending hot refrigerant from the system through it. This is the scheme, also, of Mochizuki et al (U.S. Pat. No. 4,122,688, Oct. 31, 1978).
Karlsson's brute force method of defrosting an outside evaporator (U.S. Pat. No. 3,995,809, Dec. 7, 1976) was to use not one but two heat pumps. An "auxiliary" heat pump comes on when the "base" heat pump cannot "sufficiently heat the room," and the operation of the auxiliary heat pump is reversed when the somewhat common evaporator frosts up, and room heat is sent out to melt the frost with a portion of the evaporator acting as a condenser. Hailey (U.S. Pat. No. 2,720,084, Oct. 11, 1955) found a use for evaporator ice. But it's a use that helps only a cooling system, not a heat pump. The system turns ice to advantage by using it as a heat storage mechanism, adding sections of evaporator " . . . so as to maintain substantially constant the heat transfer to the evaporator until all of the evaporator has been coated with a sufficient coating of ice so that during the heat demand the ice may be melted to provide the necessary cooling in excess of the full load rating of the compressor."
Jonsson (U.S. Pat. No. 4,065,938, Jan. 3, 1978) made a slight advancement in the fundamental heat pump art--but he still treated a symptom. His heat pump system used a basic evaporator to which was added the capacity of a booster when it could be used without raising the pressure beyond safe limits for the compressor. But the disclosure didn't include a cure for the basic problem because it still provided " . . . means for defrosting said booster heat exchanger, by periodically reversing refrigerant flow."
Anzalone (U.S. Pat. No. 4,373,353, Feb. 15, 1983) throttled refrigerant flows in a number of evaporator circuits, saying that this mode of operation resulted in a higher efficiency than obtained by previous methods that throttled flow to a single evaporator.
Such control of the working size of an evaporator is a concept old in the art; Anderson (U.S. Pat. No. 2,332,981, Oct. 26, 1943) applied the idea of a variable surface evaporator to the air conditioning system (not a heat pump) used in a railroad passenger car, to avoid excess pressure at the compressor suction.
The troublesome symptom of compressor overload in a heat pump system was addressed by Weis (U.S. Pat. No. 4,226,604, Oct. 7, 1980) for a system using a heat absorber of the solar insolation type. In such a system, the possibility of large and rapid changes in insolation aggravate the difficulty. Weis disclosed a mode of operation in which the temperature of gas leaving the evaporator was sensed and " . . . if an overheated condition is detected, bleeding off a portion of the refrigerant mixture entering the collector and mixing it with the overheated vapors to cool them down before they enter the compressor."
Adaptation of solar insolation collectors for use as heat absorbers in refrigerant cycle heat pump systems, as in Weis, required little innovative advancement. Their use as outside heat collectors was known in the solar heating system art, as in Gay (U.S. Pat. No. 4,211,209, July 8, 1980), who disclosed plates encased in transparent plastic boxes to capture solar insolation and transfer the heat to a fluid flowing through the plates, the box serving to minimize escape of heat from the plates to the atmosphere, with manifolding to equalize flow of fluid among the several plates.
A parameter in a heat pump system whose value is critical and can thus be sensed as an indicator of the need for control of evaporator size is evaporator output pressure, which is the pressure at the suction intake of the compressor. This establishes the magnitude of the compressor load. The compressor load directly affects the compressor motor's power demand, of course, so motor current is also an indicator. The patents of Weis, Jonsson and Anderson use systems that sense such parameters, a technique old in the art, evidenced, for example, in the disclosure of Plaster, (U.S. Pat. No. 3,350,897, Nov. 7, 1967). Plaster cites these parameters as signal sources for use in controlling the spin vanes of a centrifugal refrigeration compressor to adjust its capacity to the needs of a refrigeration system.
These skilled workers in the heat pump field strove to create a more satisfactory heat pump system, bringing into play the mechanical and creative skills they possessed. But there still remained several old and recognized wants: (1) The ability to pump heat in a refrigerant system without the penalty of having to defrost an evaporator, (2) The ability to pump an unfluctuating amount of heat in spite of wide fluctuations in outside ambient conditions, (3) The ability to use refrigerants to produce a colder evaporator (providing a larger temperature difference between the environment and the evaporator) so a heat pump could operate at much lower ambient temperatures, and (4) The ability to establish an operating value of --to "dial in"--the temperature at which an evaporator operates in order to create a greater temperature difference ("Delta T") between the evaporator and the ambient environment. These skilled searchers sought but failed to find a principle or mode of heat pump operation that could solve these problems.
Now, my invention affords success in these four problem categories. It does so by a new combination of old elements cooperating in a new, useful and previously unobvious manner to produce an improved result. Others skilled in the art had not devised an equivalent combination of elements and a mode of operation which would free a heat pump system from the restrictions and difficulties of these problems.